#include<stdio.h>
#include<math.h>
	int main(void)
	{  
        double a, b, c, s, area;
        scanf("%lf%lf%lf", &a, &b, &c);
        s = (a + b + c) /2;
        area = sqrt(s * (s - a) * (s- b) * (s - c));
        printf("%.3f\n", area);
        return 0;
	}
